Heat and Mass Transfer Considerations of Moist Air

This paper first presents a temperature-transforming model to simplify the analysis and calculation of heat transfer in moist air. In this model, an apparent specific heat is defined as a function of temperature, humidity ratio, enthalpy of saturated water vapor, and the specific heats of dry air and water vapor. A correction factor is then proposed to rationalize the impact of moisture on dry air specific heat. Applying this model to analyze a typical air conditioning process demonstrates that an accurate enthalpy prediction along with a clear physical essence has been achieved.

Secondly, an enthalpy-based expression of heat exchanger effectiveness is proposed to address the effects of thermal property on heat transfer analysis. A temperature-based effectiveness is then developed as a logic extension of the effectiveness discussion and the apparent specific heat method. Finally, the relationship between the current and traditional expressions of heat exchanger effectiveness is illustrated and explained.

The analytical methods developed in this paper provide a systematic approach to assessing the performance of heat exchangers under a complex operating condition where mass transfer and phase change in moist air are included. An unified chain of heat transfer analysis is established to harness the nonlinear characteristics of moist air in an effort to achieve consistency and accuracy.